1. Field of the Invention
The present invention relates to a casting or submerged entry nozzle and more particularly to a casting or submerged entry nozzle that improves the flow behavior associated with the introduction of liquid metal into a mold through a casting nozzle.
2. Description of the Related Art
In the continuous casting of steel (e.g. slabs) having, for example, thicknesses of 50 to 60 mm and widths of 975 to 1625 mm, there is often employed a casting or submerged entry nozzle. The casting nozzle contains liquid steel as it flows into a mold and introduces the liquid metal into the mold in a submerged manner.
The casting nozzle is commonly a pipe with a single entrance on one end and one or two exits located at or near the other end. The inner bore of the casting nozzle between the entrance region and the exit region is often simply a cylindrical axially symmetric pipe section.
The casting nozzle has typical outlet dimensions of 25 to 40 mm widths and 150 to 250 mm lengths. The exit region of the nozzle may simply be an open end of the pipe section. The nozzle may also incorporate two oppositely directed outlet ports in the sidewall of the nozzle where the end of the pipe is closed. The oppositely directed outlet ports deflect molten steel streams at apparent angles between 10-90xc2x0 relative to the vertical. The nozzle entrance is connected to the source of a liquid metal. The source of liquid metal in the continuous casting process is called a tundish.
The purposes of using a casting nozzle are:
(1) to carry liquid metal from the tundish into the mold without exposing the liquid metal to air;
(2) to evenly distribute the liquid metal in the mold so that heat extraction and solidified shell formation are uniform; and
(3) to deliver the liquid metal to the mold in a quiescent and smooth manner, without excessive turbulence particularly at the meniscus, so as to allow good lubrication, and minimize the potential for surface defect formation.
The rate of flow of liquid metal from the tundish into the casting nozzle may be controlled in various ways. Two of the more common methods of controlling the flow rate are: (1) with a stopper rod, and (2) with a slide gate valve. In either instance, the nozzle must mate with the tundish stopper rod or tundish slide gate and the inner bore of the casting nozzle in the entrance region of the nozzle is generally cylindrical and may be radiused or tapered.
Heretofore, prior art casting nozzles accomplish the aforementioned first purpose if they are properly submerged within the liquid steel in the mold and maintain their physical integrity.
Prior art nozzles, however, do not entirely accomplish the aforementioned second and third purposes. For example, FIGS. 19 and 20 illustrate a typical design of a two-ported prior art casting nozzle with a closed end. This nozzle attempts to divide the exit flow into two opposing outlet streams. The first problem with this type of nozzle is the acceleration of the flow within the bore and the formation of powerful outlets which do not fully utilize the available area of the exit ports. The second problem is jet oscillation and unstable mold flow patterns due to the sudden redirection of the flow in the lower region of the nozzle. These problems do not allow even flow distribution in the mold and cause excessive turbulence.
FIG. 20 illustrates an alternative design of a two-ported prior art casting nozzle with a pointed flow divider end. The pointed divider attempts to improve exit jet stability. However, this design experiences the same problems as those encountered with the design of FIG. 18. In both cases, the inertial force of the liquid metal traveling along the bore towards the exit port region of the nozzle can be so great that it cannot be deflected to fill the exit ports without flow separation at the top of the ports. Thus, the exit jets are unstable, produce oscillation and are turbulent.
Moreover, the apparent deflection angles are not achieved. The actual deflection angles are appreciably less. Furthermore, the flow profiles in the outlet ports are highly non-uniform with low flow velocity at the upper portion of the ports and high flow velocity adjacent the lower portion of the ports. These nozzles produce a relatively large standing wave in the meniscus or surface of the molten steel, which is covered with a mold flux or mold powder for the purpose of lubrication. These nozzles further produce oscillation in the standing wave wherein the meniscus adjacent one mold end alternately rises and falls and the meniscus adjacent the other mold end alternately falls and rises. Prior art nozzles also generate intermittent surface vortices. All of these effects tend to cause entrainment of mold flux in the body of the steel slab, reducing its quality. Oscillation of the standing wave causes unsteady heat transfer through the mold at or near the meniscus. This effect deleteriously affects the uniformity of steel shell formation, mold powder lubrication, and causes stress in the mold copper. These effects become more and more severe as the casting rate increases; and consequently it becomes necessary to limit the casting rate to produce steel of a desired quality.
Referring now to FIG. 17, there is shown a nozzle 30 similar to that described in European Application 0403808. As is known to the art, molten steel flows from a tundish through a valve or stopper rod into a circular inlet pipe section 30b. Nozzle 30 comprises a circular-to-rectangular main transition 34. The nozzle further includes a flat-plate flow divider 32 which directs the two streams at apparent plus and minus 90xc2x0 angles relative to the vertical. However, in practice the deflection angles are only plus and minus 45xc2x0. Furthermore, the flow velocity in outlet ports 46 and 48 is not uniform. Adjacent the right diverging side wall 34C of transition 34 the flow velocity from port 48 is relatively low as indicated by vector 627. Maximum flow velocity from port 48 occurs very near flow divider 32 as indicated by vector 622. Due to friction, the flow velocity adjacent divider 32 is slightly less, as indicated by vector 621. The non-uniform flow from outlet port 48 results in turbulence. Furthermore, the flow from ports 46 and 48 exhibit a low frequency oscillation of plus and minus 20xc2x0 with a period of from 20 to 60 seconds. At port 46 the maximum flow velocity is indicated by vector 602 which corresponds to vector 622 from port 48. Vector 602 oscillates between two extremes, one of which is vector 602a, displaced by 65xc2x0 from the vertical and the other of which is vector 602b, displaced by 25xc2x0 from the vertical.
As shown in FIG. 17a, the flows from ports 46 and 48 tend to remain 90xc2x0 relative to one another so that when the output from port 46 is represented by vector 602a, which is deflected by 65xc2x0 from the vertical, the output from port 48 is represented by vector 622a which is deflected by 25xc2x0 from the vertical. At one extreme of oscillation shown in FIG. 17a, the meniscus M1 at the left-hand end of mold 54 is considerably raised while the meniscus M2 at the right mold end is only slightly raised. The effect has been shown greatly exaggerated for purposes of clarity. Generally, the lowest level of the meniscus occurs adjacent nozzle 30. At a casting rate of three tons per minute, the meniscus generally exhibits standing waves of 18 to 30 mm in height. At the extreme of oscillation shown, there is a clockwise circulation C1 of large magnitude and low depth in the left mold end and a counter-clockwise circulation C2 of lesser magnitude and greater depth in the right mold end.
As shown in FIGS. 17a and 17b, adjacent nozzle 30 there is a mold bulge region B where the width of the mold is increased to accommodate the nozzle, which has typical refractory wall thicknesses of 19 mm. At the extreme of oscillation shown in FIG. 17a, there is a large surface flow F1 from left-to-right into the bulge region in front of and behind nozzle 30. There is also a small surface flow F2 from right-to-left toward the bulge region. Intermittent surface vortices V occur in the meniscus in the mold bulge region adjacent the right side of nozzle 30. The highly non-uniform velocity distribution at ports 46 and 48, the large standing waves in the meniscus, the oscillation in the standing waves, and the surface vortices all tend to cause entrainment of mold powder or mold flux with a decrease in the quality of the cast steel. In addition, steel shell formation is unsteady and non-uniform, lubrication is detrimentally affected, and stress within mold copper at or near the meniscus is generated. All of these effects are aggravated at higher casting rates. Such prior art nozzles require that the casting rate be reduced.
Referring again to FIG. 17, the flow divider may alternately comprise an obtuse triangular wedge 32c having a leading edge included angle of 156xc2x0, the sides of which are disposed at angles of 12xc2x0 from the horizontal, as shown in a first German Application DE 3709188, which provides apparent deflection angles of plus and minus 78xc2x0. However, the actual deflection angles are again approximately plus and minus 45xc2x0; and the nozzle exhibits the same disadvantages as before.
Referring now to FIG. 18, nozzle 30 is similar to that shown in a second German Application DE 4142447 wherein the apparent deflection angles are said to range between 10 and 22xc2x0. The flow from the inlet pipe 30b enters the main transition 34 which is shown as having apparent deflection angles of plus and minus 20xc2x0 as defined by its diverging side walls 34c and 34f and by triangular flow divider 32. If flow divider 32 were omitted, an equipotential of the resulting flow adjacent outlet ports 46 and 48 is indicated at 50. Equipotential 50 has zero curvature in the central region adjacent the axis S of pipe 30b and exhibits maximum curvature at its orthogonal intersection with the right and left sides 34c and 34f of the nozzle. The bulk of the flow in the center exhibits negligible deflection; and only flow adjacent the sides exhibits a deflection of plus and minus 20xc2x0. In the absence of a flow divider, the mean deflections at ports 46 and 48 would be less than xc2xc and perhaps ⅕ or 20% of the apparent deflection of plus and minus 20xc2x0.
Neglecting wall friction for the moment, 64a is a combined vector and streamline representing the flow adjacent the left side 34f of the nozzle and 66a is a combined vector and streamline representing the flow adjacent the right side 34c of the nozzle. The initial point and direction of the streamline correspond to the initial point and direction of the vector; and the length of the streamline corresponds to the length of the vector. Streamlines 64a and 66a of course disappear into the turbulence between the liquid in the mold and the liquid issuing from nozzle 30. If a short flow divider 32 is inserted, it acts substantially as a truncated body in two dimensional flow. The vector-streamlines 64 and 66 adjacent the body are of higher velocity than the vector-streamlines 64a and 66a. Streamlines 64 and 66 of course disappear into the low pressure wake downstream of flow divider 32. This low pressure wake turns the flow adjacent divider 32 downwardly. The latter German application shows the triangular divider 32 to be only 21% of the length of main transition 34. This is not sufficient to achieve anywhere near the apparent deflections, which would require a much longer triangular divider with corresponding increase in length of the main transition 34. Without sufficient lateral deflection, the molten steel tends to plunge into the mold. This increases the amplitude of the standing wave, not by an increase in height of the meniscus at the mold ends, but by an increase in the depression of the meniscus in that portion of the bulge in front of and behind the nozzle where flow therefrom entrains liquid from such portion of the bulge and produces negative pressures.
The prior art nozzles attempt to deflect the streams by positive pressures between the streams, as provided by a flow divider.
Due to vagaries in manufacture of the nozzle, the lack of the provision of deceleration or diffusion of the flow upstream of flow division and to low frequency oscillation in the flows emanating from ports 46 and 48, the center streamline of the flow will not generally strike the point of triangular flow divider 32 of FIG. 18. Instead, the stagnation point generally lies on one side or the other of divider 32. For example, if the stagnation point is on the left side of divider 32 then there occurs a laminar separation of flow on the right side of divider 32. The separation xe2x80x9cbubblexe2x80x9d decreases the angular deflection of flow on the right side of divider 32 and introduces further turbulence in the flow from port 48.
Accordingly, it is an object of our invention to provide a casting nozzle that improves the flow behavior associated with the introduction of liquid metal into a mold through a casting nozzle.
Another object is to provide a casting nozzle wherein the inertial force of the liquid metal flowing through the nozzle is divided and better controlled by dividing the flow into separate and independent streams within the bore of the nozzle in a multiple stage fashion.
A further object is to provide a casting nozzle that results in the alleviation of flow separation, and therefore the reduction of turbulence, stabilization of exit jets, and the achievement of a desired deflection angle for the independent streams.
It is also an object to provide a casting nozzle to diffuse or decelerate the flow of liquid metal traveling therethrough and therefore reduce the inertial force of the flow so as to stabilize the exit jets from the nozzle.
It is another object to provide a casting nozzle wherein deflection of the streams is accomplished in part by negative pressures applied to the outer portions of the streams, as by curved terminal bending sections, to render the velocity distribution in the outlet ports more uniform.
A further object is to provide a casting nozzle having a main transition from circular cross-section containing a flow of axial symmetry, to an elongated cross-section with a thickness which is less than the diameter of the circular cross-section and a width which is greater than the diameter of the circular cross-section containing a flow of planar symmetry with generally uniform velocity distribution throughout the transition neglecting wall friction.
A still further object is to provide a casting nozzle having a hexagonal cross-section of the main transition to increase the efficiency of flow deflections within the main transition.
A still further object is to provide a casting nozzle having diffusion between the inlet pipe and the outlet ports to decrease the velocity of flow from the ports and reduce turbulence.
A still further object is to provide a casting nozzle having diffusion or deceleration of the flow within the main transition of cross-section to decrease the velocity of the flow from the ports and improve the steadiness of velocity and uniformity of velocity of streamlines at the ports.
A still further object is to provide a casting nozzle having a flow divider provided with a rounded leading edge to permit variation in stagnation point without flow separation.
A still further object is to provide a casting nozzle which more effectively utilizes the available space within a bulged or crown-shaped mold and promotes an improved flow pattern therein.
A still further object is to provide a casting nozzle having a bore with a multi-faceted interior geometry which provides greater internal cross-sectional area for the bore near a central axis of the casting nozzle than at the edges.
A still further object is to provide a casting nozzle which achieves a wide useful range of operational flow throughputs without degrading flow characteristics.
A still further object is to provide a casting nozzle with baffles which proportion the flow divided between outer streams and a central stream so that the effective discharge angle of the outer streams exiting upper exit ports varies based on the throughput of liquid metal through the casting nozzle.
A still further object is to provide a casting nozzle with baffles which proportion the flow divided between outer streams and a central stream so that the effective discharge angle of the outer streams exiting upper exit ports increases as the throughput of liquid metal through the casting nozzle increases.
It has been found that the above and other objects of the present invention are attained in a method and apparatus for flowing liquid metal through a casting nozzle includes an elongated bore having at least one entry port, at least one upper exit port, and at least one lower exit port. A baffle is positioned proximate to the upper exit port to divide the flow of liquid metal through the bore into at least one outer stream and a central stream, the outer stream flowing through the upper exit port and the central stream flowing past the baffle and toward the lower exit port. The baffle is adapted to allocate the proportion of liquid metal divided between the outer stream and the central stream so that the effective discharge angle of the outer stream exiting through the upper exit port varies based on the flow throughput of liquid metal through the casting nozzle.
Preferably, the effective discharge angle of the outer streams increases as flow throughput increases.
In a preferred embodiment, the baffles are adapted so that about 15-45%, most preferably 25-40%, of the total flow of liquid through the casting nozzle is allocated to the outer streams and about 55-85%, most preferably 60-75%, of the total flow of liquid through the nozzle is allocated to the central stream.
In a preferred embodiment, the theoretical discharge angle of the upper exits ports is about 0-25xc2x0, and most preferably about 7-10xc2x0, downward from the horizontal.
The casting nozzle may also include a central axis and at least one entry port and at least one exit port, the bore of the casting nozzle including an enlarged portion to provide the bore with greater cross-sectional area near the central axis than near the edges of the bore.
In a preferred embodiment, the enlarged portion comprises at least two bending facets, each of which extends from a point on a plane which is substantially parallel to and intersects the central axis, toward a lower edge of the bore. In a preferred embodiment, the bending facets include a top edge and a central edge, and at least two of the top edges are adjacent to each other to form a pinnacle pointing generally toward the entry port. Preferably, the central edge of each bending facet is more distant from a lengthwise horizontal axis of the casting nozzle than the top edge of the bending facet within a horizontal cross-section.
It has been found that the above and other objects of the present invention are attained in a method and apparatus for flowing liquid metal through a casting nozzle that includes an elongated bore having an entry port and at least two exit ports. A first baffle is positioned proximate to one exit port and a second baffle is positioned proximate to the other exit port.
The baffles divide the flow of liquid metal into two outer streams and a central stream, and deflect the two outer streams in substantially opposite directions. A flow divider positioned downstream of the baffles divides the central stream into two inner streams, and cooperates with the baffles to deflect the two inner streams in substantially the same direction in which the two outer streams are deflected.
Preferably, the outer and inner streams recombine before or after the streams exit at least one of the exit ports.
In a preferred embodiment, the baffles deflect the outer streams at an angle of deflection of approximately 20-90xc2x0 from the vertical. Preferably, the baffles deflect the outer streams at an angle of approximately 30xc2x0 from the vertical.
In a preferred embodiment, the baffles deflect the two inner streams in a different direction from the direction in which the two outer streams are deflected. Preferably, the baffles deflect the two outer streams at an angle of approximately 45xc2x0 from the vertical and deflect the two inner streams at an angle of approximately 30xc2x0 from the vertical.
Other features and objects of our invention will become apparent from the following description of the invention which refers to the accompanying drawings.